When updating revisions in response to commits, reuse previously generated diffs
Fixes T10968. In rare situations, we can generate a diff, then hit an error which causes this update to fail.
When it does, we tend to get stuck in a loop creating diffs, which can fill the database up with garbage. We saw this once in the Phacility cluster, and one instance hit it, too.
Instead: when we create a diff, keep track of which commit we generated it from. The next time through, reuse it if we already built it.
- Used bin/differential attach-commit <commit> <revision> to hit this code.
- Simulated a filesystem write failure, saw the diff get reused.
- Also did a normal update, which worked properly.
Reviewed By: chad
Maniphest Tasks: T10968
Differential Revision: https://secure.phabricator.com/D17164